ISO is an algorithmic value that indicates a film stock’s sensitivity to light. Since dSLRs use a CCD or CMOS image sensor instead, the ISO on a digital camera is the equivalent in terms of light sensitivity. Digital camera makers decided there was no reason to define a new measurement system, so ISO was carried over to the digital realm.

ISO is a numeric value that helps determine, along with the aperture and shutter speed, the exposure length needed to acquire an effective photo and accurately reproduce the subjects in your viewfinder. A rule of thumb: The higher the ISO number the more sensitive the image sensor is and the shorter the exposure needs to be. The opposite is also true.

Just like with film, the ISO setting also indicates how much grain will be apparent in the final photograph. When you choose a high ISO, you’re raising the image sensor sensitivity and that results in more electronic noise showing up in the image. Noise is any light signal that doesn’t originate from your subject and is the bane of digital photography.

The grain in film can sometimes be used for artistic purposes. Digital camera engineers have designed the image sensor (CCD or CMOS) to perform best at the lowest ISO rating. So the trick with a digital camera is choosing one that has acceptable noise at the higher ISO ratings.

If you want smooth, appealing and color-accurate photos, then use a lower ISO setting. However, if you’re shooting in low light conditions or want to stop fast-moving action then you’ll want to increase your ISO so your shutter speed can be higher. That way you still get the correct exposure.

It’s rare for a dSLR to have an ISO-priority feature, but some do have it. This is when you set the ISO and the camera selects the best shutter speed and aperture. Then you can adjust the ISO to suit the needs of what you’re shooting and the camera compensates.

Something to take note of, the actual size of your camera’s image sensor determines what ISO rating the camera can acceptably use without having electronic noise invade your photo. So newer dSLRs with sensors the same size as a 35mm film frame will have better performing ISO ratings than a small sensor but with the same megapixel value.

ISO is a key component in understanding how to obtain the best exposure without injecting unwanted noise into your image. You can remove some noise either with in-camera technology or in an image processor like Photoshop. But, only so much can be removed and it requires mapping out each of the colors (Red, Green & Blue).

The thing about digital cameras is that the engineers actually designed the camera sensors to perform optimally and most robustly at the lowest possible ISO for each camera. This is typically ISO 100.

Master the ISO setting and you’ll be on your way to better digital photos.